The present invention relates to patient monitoring, and more specifically to acquiring oximetry and electrocardiogram signals.
Conventional oximeter sensors are generally only capable of providing blood oxygen saturation data. However, oximeter sensors are often used at the same time that an electrocardiogram is being acquired. Electrocardiogram instruments require many design considerations, as described in more detail below. Conventionally, these design considerations are addressed with electrodes and an ECG instrument that are independent of the oximeter sensor.
As used herein, the term “ECG” refers to a standard ten-electrode, twelve-lead electrocardiogram or any non-standard electrocardiogram, as are known to one of ordinary skill in the art. The term “ECG signal” or “ECG output signal” refers to one or more leads or channels of data. The term “channel of ECG data” refers to a single lead or a single channel. The term “ECG instrument” refers to any acquisition, analysis, signal generation, and/or monitoring equipment that receives ECG signals from electrodes.
An ECG instrument measures voltage potentials between electrodes or combinations of electrodes attached to a patient. In a minimum configuration, one channel of ECG data is measured between two electrodes. To obtain more channels, additional electrodes are needed. Typically, the voltage between the electrodes (i.e., the differential voltage) is relatively low in amplitude (e.g., approximately one mV). Accordingly, high amplification is needed by the ECG instrument to raise the differential voltage. The body of the patient often has a common mode voltage that is very high in comparison to the differential voltage. It is understood by those of ordinary skill in the art that a high common mode rejection ratio (“CMRR”) and a high input impedance are required for an ECG instrument to amplify a low differential voltage in the presence of large common mode voltages in order to acquire an ECG signal of adequate diagnostic quality [i.e., adequate voltage resolution, signal-to-noise ratio (“SNR”), and frequency bandwidth].
For applications in which an ECG signal of reduced quality is adequate (e.g., less than about eight bit resolution for a heart rate monitor in an exercise bicycle, a defibrillator, or an event recorder), the lower frequencies (e.g., less than approximately 0.6 Hz) and the higher frequencies (e.g., greater than approximately 40 Hz) can be filtered from the ECG signal. For these applications, the heart rate monitor may use only two electrodes and a relatively low-cost amplifier.
To avoid the high cost and difficult design of a high CMRR amplifier, an ECG instrument can include a third electrode with a low impedance path between the patient and the ECG instrument. The third electrode is generally referred to as the right leg (“RL”) electrode, the common electrode, or the reference electrode. The reference electrode does not serve as a positive input or a negative input to the ECG amplifier(s). Only one reference electrode is necessary, even if many other electrodes are used to acquire several other channels of ECG data. The reference electrode can be positioned on the patient's body in any suitable manner known in the art.
When monitoring infants, especially neonates, the application of many adhesive electrodes to the skin is particularly undesirable. The body surface of an infant is often so small that electrodes can cover relatively large areas of the infant's body. The skin of a neonate is extremely fragile and removing electrodes can easily tear the skin increasing infection risk and pain to the infant. Each electrode also requires a leadwire to connect the electrode to the ECG instrument. Additional electrodes and leadwires decrease the caregiver's ability to access the infant. Additional electrodes also add cost in materials and labor, and increase the time it takes to prepare an infant for monitoring.
Conventional patient monitors often acquire ECG signals, an impedance respiration signal, and a pulse oximeter signal from a patient. The combination of these three monitoring parameters is generally referred to as a cardio-respirogram (“CRG”) signal or a pneumogram signal. The CRG signal is of special value in the care of a neonate. To acquire CRG signals, the patient monitor is generally connected to three electrodes for acquiring one channel of ECG data and an impedance respiration signal. A separate transducer, the pulse oximeter sensor, is often attached to an ear lobe, a finger, a toe or another body surface. The pulse oximeter sensor generally illuminates the skin with two colors of light (e.g., red and infra-red) and measures the reflected or transmitted light to determine the level of oxygen saturation of the blood.